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Friday, April 5, 2013

Emu: A large bird with surprisingly intact sex chromosomes

Emu are one of the largest living species of birds:

Awesome emu.

They lay large, blue eggs and have little stripped babies:

Without arms, emu hug with their minds, or I like to think that they do.

Like mammals, sex is determined in birds using sex chromosomes, but in birds it is a little different. In mammals (dogs, humans, cows) females have two copies of the same sex chromosome (XX), and males have one X, and one Y, where genes on the Y chromosome turn on the pathways for male features. In birds, it is the males who have two copies of the same sex chromosome (here we call them ZZ), and females who have one Z chromosome and one W chromosome. In birds, male-specific features require expression (product) from two copies of a gene (so males have two Z chromosomes).

W and Y are usually much smaller than their partners

What you'll notice in the picture above is that in both mammals and birds one of the sex chromosomes (X or Z) is large, while the other (Y or W) is small. Generally one of the sex chromosomes becomes sex-specific (such as the Y passed down through the male lineage in mammals, or the W passed through the female lineage in birds). As this chromosome becomes sex-specific, it will accumulate genes, and functions, that are beneficial to one sex, and neutral, or even harmful to the other. Usually all chromosome pairs can swap bits of DNA (also called recombination), but to prevent these sex-specific genes from acting in the opposite sex (where they would do harm), the sex chromosomes usually stop swapping DNA. But, these swaps between partners can also serve as a bandage to fix errors that happen (think, having a partner to remove that broccoli you didn't know was lodged in your teeth, but instead of broccoli, it is an error in a gene). A drawback of stopping the swaps is that without a partner to check and make sure everything is working properly, the W and Y chromosomes start accumulating mutations, losing genes, and shrinking.

But, when Vicoso, Kaiser and Bachtrog looked at the emu sex chromosomes, they saw something really amazing. Whereas the W in most birds is small and degraded (like the human Y), the W in emu is quite large, nearly the same size as it's partner.

Emu W has nothing to prove to you.

This is pretty unusual among bird sex chromosomes, and the authors wanted to figure out why. So, they looked at how the genes on the sex chromosomes were working in males and females. Although sometimes we think of genes as being "on" or "off", in reality many genes are "on", but they are doing their jobs (making proteins) with a higher or lower efficiency (think of your heart; if it were ever truly "off" you'd be dead, but there are times when your heart is pumping slowly, and times when it is racing, and it is the level at which your heart is working that is important). So, looking at genes on the emu Z and W, they saw that many of the genes were being used at much higher levels in males than in females. This suggests that there is some mechanism by which genes on the emu sex chromosomes have evolved to favor functions in males over females, which side-steps the usual path of sex chromosome evolution that would lead to a small, degraded W. Well-played, emu, well-played.

I think that's true, John. You may be dissatisfied with all of my posts. I am trying to write for a more general audience, so will have many simplifications. I am aiming to reach a balance between content, and accessibility.

Hey, I just like to quibble. Don't mind me. I think you're giving the unintended impression that a) homomorphic sex chromosomes are a new discovery and b) that discovery was limited to emus, rather than that the authors propose a mechanism for a more widespread phenomenon based on study of emus.

Quibbling is welcome, and definitely not taken personally, but I don't know how often I'll have to acknowledge.

I would like to argue, though, that to most people it will be news that homomorphic sex chromosomes exist, and in this paper, they do focus on emus (not the rest of birds with homomorphic sex chromosomes).

Not all of the articles I blog about are recent either. I think it is more important to tell an accurate story to the public to engage them in science, rather than ignore them, or drown them in vocabulary, like most scientists do.

I can't access the paper at home (it isn't open access), so maybe Beatriz can correct me if I'm wrong. But, I think that a large portion of the sex chromosomes in Emu still undergo recombination, but that there is at least one region that stopped recombining a long time ago (near DMRT1) and at least one region where recombination is thought to have stopped more recently.

Just in case my original post actually made it to your inbox or was seen, I want to apologize profusely - I am commenting from my tablet and my auto correct made a very inappropriate correction. Below is my original comment corrected:

I came across your post from PT which I read casually and never comment on, but just wanted to come by and commend your post and your work.

I hold an undergrad degree in evo bio and am currently in my final year of medical school and do clinical effectiveness research so the deeper aspects of evo bio are relegated to my hobby interest. However, I find my background and continued reading to be extremely helpful in understanding and applying medical knowledge.

Posts like these I think are particularly useful - you do an excellent job of distilling down complex concepts in a manner which not only makes them more accessible to the average lay person, but also to someone like myself to think about the application to medical science.

I tutor the underclass students in medical board prep and being able to relate mechanisms and fundamental ideas like this in an easy and relevant way makes for easier understanding and better retention.

My most recent tutorial was in oncogenesis and medical genetics so I was actually able to use this tidbit from your post as a one liner to help the students understand a bit better.

Once again apologies for the typo. It would have been quite funny if not for how appropriately inappropriate it was (if you understand what I mean by that). I was actually rather horrified since I consider myself a feminist and staunchly and avidly support women in science - my own fiancée is a NASA engineer.

As for clinicians being interested in genetics... Yes, more and more but not as much as I (and I imagine you) would like. Obviously we have an entire sub specialty of medical genetics but I think the average clinician (who is not a scientist as medical school does not teach you to be one) do relegate it to "another class to pass."

However with the burgeoning fields of epigenetics and proteomics I believe these concepts are increasingly relevant, especially in the context of providing more effective and individually personalized medical care (the real kind of personalized medical care, not the alternative medicine BS).

I like to make it fun and interesting and the way to win the hearts and minds of young medical students is to show them how such knowledge can help them remember more and score better on our board exams (I think we should do it for deeper reasons obviously, but I am also an outcomes oriented pragmatist, lol).

In any event, I just wanted to take the time to comment in support of your efforts for all the reasons I mentioned above, and especially sinceI know that it is always more common to garner criticism and from my own personal experience (I write a lot about CAM and pseudoscience) the occasional positive comment provides fuel and motivation for continued efforts.

Even though i likely won't comment much, if at all, I'll certainly keep my eye on your posts (checked that one carefully for any autocorrect errors! ) and shamelessly steal your wonderful tidbits to include in my teaching sessions (with attribution, where appropriate of course).

It's Logan's mom ... so interesting!! IDK if he mentioned that I have a flock of parakeets and 3 african greys, a caique and 3 cockatiels ... I am swimming in bird ... =) Love your blog and will keep checking back!

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